This application is the national phase under 35 U.S.C. xc2xa7371 of PCT International Application No. PCT/EP01/01014 which has an International filing date of Jan. 31, 2001, which designated the United States of America and which claims priority on European Patent Application No. 001 04 001.3 filed Feb. 25, 2000, the entire contents of which are hereby incorporated herein by reference.
The invention generally relates to a device for casting a workpiece. In particular, it relates to an internally cooled turbine blade or vane, having a casting cavity in which casting cores, which produce passages which pass through the workpiece, are present. Further, it generally relates to a process for casting a workpiece.
Internally cooled turbine blades or vanes which arc exposed to hot gas are often cooled by what is known as film cooling. In film cooling, cooling air flows out of the interior of the blade or vane profile through bores. A film of air which has a cooling action forms on the outer side of the outer wall of the blade or vane profile. The bores are either cast in directly or arc drilled out at a later stage. To produce the cast bores, cylindrical casting cores, which are matched to the continuous passages, arc secured in the two casting-mold parts which form the inner and outer sides of the outer wall. Therefore, bores with a large bore diameter which lie a relatively long way apart are formed.
Consequently, there are relatively poorly cooled regions everywhere between the film-cooling bores. This fact is compensated for by using a greater flow of coolant than is actually required, in order for these relatively poorly cooled regions also to be sufficiently cooled.
An object of an embodiment of the present invention can be to propose a device for casting a workpiece without poorly cooled regions. In particular, it may be directed to casting a workpiece using internally cooled turbine blades or vanes, in order to create the possibility of sufficient film cooling with a low consumption of coolant. An object of an embodiment of the invention may be achieved by introducing casting cores into the casting cavity in such a manner that they rest loosely against one another.
The fact that the casting cores rest loosely against one another produces a dense packing of casting cores which differs according to the shape and size of the casting cores. Casting material which has been introduced is displaced at the points of contact between the casting cores. After casting, the casting-core material is removed from the material by chemical means, for example by leaching. Passages which pass through the workpiece and are virtually randomly distributed through the region which was filled with casting cores are formed, the channel density in a predetermined relationship with the casting-core density according to the size and shape of the casting cores.
The passages have openings on both sides of the workpiece. With the casting cores resting loosely against one another, virtually every casting core has at least one neighbor with which it is in contact. Further, the latter in turn has a further neighbor, and so on, until a casting core connected thereto is in contact with the other outer side of the workpiece.
In this way, it is possible for even casting materials which are able to withstand high temperatures to be processed for the production of film-cooled turbine blades or vanes, but also for cover plates and heat shields. The choice of a small size of casting core and a suitable shape of casting core produces a very large number of small channel outlet openings.
A passage which passes through the workpiece generally has a plurality of openings which lie closely together as outlets. If film cooling is used in a workpiece of this type, all regions of the surface, which is interrupted by openings of the passages, can be reached by the cooling film. At the same time, the workpiece which has passages passing through it is sufficiently strong, on account of the strong casting material but also on account of a specific choice of the shape and size of the casting cores, as will be explained in more detail below. Production of the casting device is simplified, since the casting cores which are introduced in this manner are supported against one another and therefore do not have to be secured separately in the surrounding casting-mold walls.
The fact that the casting cores are resting against one another indicates that, after casting, suitable passages with a small diameter at the passage opening are formed when at least two casting cores rest against one another producing a passage which passes through the workpiece.
If the maximum external dimensions of the casting cores are smaller than the minimum internal dimensions of the casting cavity, it is ensured that at least two or more casting cores can be distributed over the cross section of the casting cavity, in contact with one another, at any location in the casting cavity. In this way, it is possible to produce very small, branched passage structures depending on the size, shape and packing density of the casting cores.
The setting up of the casting mold as a whole is significantly simplified if the casting cores can be poured into the casting cavity. Even narrow, angular regions of the casting mold can in this way be occupied by the casting cores.
If the casting cores are approximately circular and/or ellipsoidal, they are easy to pour and are distributed well through the casting mold, without leaving large free spaces clear. The casting cores have a large surface area in order to produce contact locations with other, adjoining casting cores, so that a high passage density is produced in the cast workpiece. With ellipsoidal casting cores, it is possible in particular to produce elongate passage sections with a high passage density if the contact locations lie predominantly at the largest transverse dimensions of the ellipsoids.
If the casting cores are of approximately equal size, it is in this way possible to produce highly uniform passage structures which can be successfully predetermined.
If the diameters of the casting cores are between approximately 0.1 and approximately 2 mm, it is possible, in particular for standard turbine blade or vane wall thicknesses, to produce a number of passages which is sufficient for optimum film cooling. Therefore, the casting cores of this type are neither too small, which could entail casting problems, nor too large, meaning that cooling of the workpiece can only be achieved with a high level of coolant.
If the casting cores have hollows which can be filled with casting material, the workpiece is sufficiently strong despite its porous structure. On account of the hollows, the casting cores have a surface area which is large by comparison with its volume. Consequently, the proportion of casting material in the cast workpiece is increased.
If the hollow is a bore and runs through the center of the casting core, particularly good strength of the workpiece is produced even locally in the region of each core, since the casting cores are leached out after casting and in each case at least one central strut formed by the material remains in place, ensuring sufficient strength.
If only a predetermined part of the workpiece casting mold is filled with casting cores, part of the workpiece can be designed with passages and another part can be of solid design. This option can be used in particular in turbine blades or vanes as a result of the casting cores being introduced only into the region of the casting mold which produces the outer walls. In this case, only an outer wall has an open porosity, while the remainder of the blade or vane has the casting material in its original form. The outer wall can then be cooled by means of consumption-optimized film cooling. If the casting cores arc compacted using a vibratory device, it is possible to produce very narrow passage structures even with irregular casting cores. In this way, it is possible to correct uneven filling of the casting mold.
The casting cores can be prevented from floating up during casting if the casting cores which have been introduced into the casting cavity are held together.
If the casting cores are held together by meshes, on the one hand the casting cores are prevented from floating up during casting, and on the other hand, at the same time, it is possible to collect any slag materials which accumulate on the casting material. For this purpose, the mesh width must on the one hand be smaller than the diameter of the casting cores but on the other hand must be large enough to allow the slag materials to pass through.
Furthermore, the size of the passages can be adjusted if the casting cores which have been introduced into the casting cavity can subsequently be coated with a material which is able to withstand casting and bonds to them. The casting-resistant material bonds both to the surface and also in particular to the contact locations between two casting cores. In this way, these contact locations are reinforced and acquire a large diameter, which in turn influences the passage diameter. Furthermore, the applied material makes it possible to form additional contact locations if casting cores previously lay very close together but were not in contact with one another. Furthermore, the casting cores are held together better by the coating and the casting cores are prevented from floating up in the casting material.
If the casting mold is connected to an evacuation device, during casting the casting material is drawn even into the smallest cavities in the casting mold, in particular between the casting cores. The formation of regions which are free of casting material is avoided. Moreover, the casting operation is accelerated. The use of retaining devices, for example meshes, prevents casting cores from being drawn toward the evacuation device together with the casting material. To improve a casting process, it is proposed for casting cores to be introduced into the casting cavity in such a manner that they rest loosely against one another. This process produces, in a simple manner, continuous passages whose dimensions can easily be changed by suitable selection of the dimensions of the casting cores and the production of which does not require any complex preparation of the casting mold.
If the casting cores which have been introduced into the casting cavity are subsequently coated with a material which is able to withstand casting and bonds to them, they are held in the mold without it being necessary to use complex devices. Depending on the target size of the passages, the coating process can be repeated a number of times, in order in this way to improve cohesion between the cores or to produce new connections.
To improve a workpiece, it is proposed for passages to pass through the workpiece in the form of a three-dimensional grid. A workpiece of this type can be cooled sufficiently as a result of cooling air being passed through it on the other side even if the flow of cooling air is low. In the three-dimensional grid arrangement, the passages, the diameters of which vary as a function of the shape and arrangement of the casting cores, generally have multiple branches and a plurality of openings.
Very good workpiece properties are achieved if practically a quarter of the entire area of a workpiece side is made up of the area of uniformly distributed passage openings. On the one hand, there is then scarcely any location on a side of the workpiece which has passages passing through it which is relatively poorly cooled, since in the case of film cooling a well-cooled region which amounts to three times the width of the passage opening is formed behind the passage opening. Therefore, all regions of the workpiece side which has passages passing through it are cooled uniformly when the passage opening area forms a quarter of the total area. At the same time, the workpiece is sufficiently strong even in the region which has passages passing through it.
If the passage openings have diameters of between approximately 0.1 and approximately 2 mm, optimum cooling in particular of an outer wall, which has passages passing through it, of a conventional, internally cooled turbine blade or vane is ensured with film cooling.